Testing DRAM From a Commodore 64

A few months ago, [Josh] was given an old Commodore 64. He needed to make an AV cable and find a new power supply, and even after testing these new parts out, [Josh] found it still wouldn’t boot. Not one to look a gift horse in the mouth — or perhaps he enjoys the challenge — he set out on restoring a thirty year old circuit board.

He replaced a few chips and the caps, but found he had no way to test the DRAM chips. Compared to SRAM or Static RAM used by other computers of the era, DRAM is a bit harder to interface, requiring a capacitor in each memory cell to be refreshed a few dozen times every second. With a bit of help from his good friend [CNLohr], [Josh] figured out a circuit to read and write to his chips and build a small board based on the ATmega8U2 microcontroller for testing purposes.

I have been wanting to build a new motherboard for the Commodore 64 using modern components (but original custom chips) for years. I really need to just sit down and do it.

The idea is to use an FPGA (probably a CPLD would work fine) to replace the glue logic, use SRAM and flash for the roms, and of course modern storage like SD cards. The goal being to reduce the size into a portable, but keep the authentic sound and graphics.